The Gold Coast

After driving through the mountains and seeing 400 million years of the history of Queensland in the last post, we finished the day on the beach in a tourist area known as The Gold Coast (Fig. 1).

 Fig. 1

As can be seen in Fig. 1, this is a wide beach, called dissipative. We saw a similar beach form on a low-energy beach in Tasmania, but that was not in equilibrium with wave conditions. This beach has the appropriate morphology for the unrelenting surf impacting the fine sand (Fig. 2).

 Fig. 2

Breakers on such beaches are 6-9 feet in height and construct several bars parallel to the beach. The small sand grains are easily moved and result in a relatively flat beach profile.

We noted the evidence for storms in Tasmania and potential evidence of the beaches not recovering. That is a problem here as well. Figure 2 was taken from the top of a continuous dune that faced the beach. It was about ~10 feet high in this location but varied along the beach. For example, less than 1/4 mile along the beach, it was only 6 feet (Fig. 3).

 Fig. 3

Note the erosion at the toe of the dune and the exposed roots of the small tree. Thick grasses are partially armoring the dune face but there are still signs of permanent sand loss. How severe depends on when the damage occurred. The vegetation advancing towards the beach suggests that it has been a couple of years at least. If so, this beach is eroding because sand blown off the swash zone should heal any damage within a year.

Some locations seemed to be recovering whereas others reveal scarps (Fig. 4).

 Fig. 4

Note the scarp in the foreground of Fig. 4 and the lower slope in the middle part of the photo. This apparent recovery is probably due to slumping from higher up the slope, as indicated by erosion runnels perpendicular to the beach. Further along the beach, in an undeveloped area where human impacts are minimum, the dune is high but shows evidence of multiple erosion events (Fig. 5).

 Fig. 5

Not the flatter area halfway up the dune (right side of photo), covered with grass. This location appears to have recorded a large event long before a more recent one. There is no appreciable recovery from either.

It would seem that beautiful, sandy beaches fed by multiple rivers carrying sediment from eroding mountains isn’t enough to maintain equilibrium with rising sea level. The world’s coast lines are sediment starved and cannot fight against existing sea level, much less further increases.

 

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